Basic question about ratio settings

Been shooting with (and without) flash for years! But, I just got a second flash for backgrounds and highlights, etc.
I'm reading the manuals about ratios and trying to figure out how to set things up.
Stupid question...if I have the main light as A and the secondary light as B, and the ratio is set at 8:1, for example, does that make the main light lighter or does it decrease the output of the B light? Or neither?
And, should I just leave the ratio at 1:1 and adjust the output by a couple of stops on the B flash manually?
(I tried trolling through some earlier posts using "ratio" as a search, but didn't find anything relevant to my particular question.)
Thanks...I seem to be stuck and I realize it's probably a very basic question!

sml wrote in post #13392106
Been shooting with (and without) flash for years! But, I just got a second flash for backgrounds and highlights, etc.
I'm reading the manuals about ratios and trying to figure out how to set things up.
Stupid question...if I have the main light as A and the secondary light as B, and the ratio is set at 8:1, for example, does that make the main light lighter or does it decrease the output of the B light? Or neither?
And, should I just leave the ratio at 1:1 and adjust the output by a couple of stops on the B flash manually?
(I tried trolling through some earlier posts using "ratio" as a search, but didn't find anything relevant to my particular question.)
Thanks...I seem to be stuck and I realize it's probably a very basic question!

If you are referring to the lighting ratio as used for the Canon wireless system, then a lighting ration of 8:1 means that the flash output from slave group A is 8 times brighter than that of slave group B. In terms of f-stops, group A would be 3 stop brighter than group B. As far as overall exposure is concerned, it is calculated taking into consideration the output of all the flashes in group A and group B, so that the exposure is correct. Of course, any FEC set on the camera or master slave will be taken into account after the overall exposure has been determined.

Setting a ratio of 1:1 would make the overall lighting flat. If there's a need to do that, then by all mean, set it to 1:1. But usually, you want the fill to be less bright than the main.

Thanks so much, PacAce. I appreciate your response. It helps.
But, just to clarify for me (still workin' on this!)....will the A group be lighter than if were being used as the sole light, or is the B group less output than if it would be the sole light? That is, what is the relative point---group A or B? Which light is adjusting for the ratio?
And, is it common practice to set the ratio up or rather adjust the output of each flash manually?

sml wrote in post #13392993
Thanks so much, PacAce. I appreciate your response. It helps.
But, just to clarify for me (still workin' on this!)...will the A group be lighter than if were being used as the sole light, or is the B group less output than if it would be the sole light? That is, what is the relative point---group A or B? Which light is adjusting for the ratio?
And, is it common practice to set the ratio up or rather adjust the output of each flash manually?

That's a good question and the answer is that the light output will be the same whether the flash groups are used together or individually. In other words, for example, group B flash will output the same light whether group A flash is present or not, so, for the 8:1 ratio you used as an example, its output will always be 3 stops less than what group A would be even if group A is not present.

Just a side note, the above is true in theory but in practice, the individual light output may not be exactly the same if only one group is used but ratio is specified. This is because the overall exposure on which the final lighting ratio is based on is determined by firing all the flashes during pre flash. If group A, for example, does not exist, then the overall exposure will be based on the pre flash of group B only.

Ah! I think get it now!
The ratio is more the overall exposure. It doesn't "underexpose" or "overexpose."
So, if I want to underexpose one of the flash units (for example), that would be a separate thing to do...and I could still use the ratio for concentrating the light where I want it.
And, for another example, I would likely under or overexpose if I used a grid or other flash "attachment" rather than control that with the ratio setting.
Thanks for your responses!

The confusion in the numbers behind the ratios is that contrast ratios are expressed in two different manners...1) ratio of light source outputs, and 2) contrast ratio of light falling upon the subject. Most photographers indeed fail to differentiate ratio 1 from ratio 2 when they talk about lighting, but as you can see from the three things discussed below, it can make a difference in the outcome, both numerically in the ratio and also photographically in the relative contrastiness of the lighting!

Examples of both, using the same starting assumptions: Source B (Fill) alone measures f/4, Source A (Key) measures f/8

1. Light source ratio: f/8 : f/4 is 2EV difference A:B, or '4:1' source ratio
   
2. Subject contrast ratio (situation 1, where Fill falls on one half of surface seen by lens, Key falls on other half only):
   Fill side gets '1 part' of light, Key side gets '4 parts' of light,
   subject contrast ratio = '4:1'
   
3. Subject contrast ratio (situation 2, where Fill falls on entire surface seen by lens, Key falls on one side one):
   Fill side gets '1 part' of light, Key side gets '1part' from the Fill plus '4 parts' of light from the Key,
   subject contrast ratio = '5:1'
   

Thanks.
Good explanations.